Various lithographic printing plates requiring no fountain solution for performing lithographic printing using no fountain solutions have been proposed. In particular, plates comprising a substrate having thereon a photosensitive layer and an ink-repellent layer have excellent characteristics. For example, various plates, such as positive, negative, ink-repellent upper layer, and ink-repellent lower layer types, have been proposed. Specifically, the positive ink-repellent upper layer type plates are described in JP-B-44-23042 (the term "JP-B" as used herein means an "examined Japanese patent publication") and JP-B-54-26923, and the positive ink-repellent lower layer type plates are described in JP-A-64-74541 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and JP-A-1-149043. Furthermore, the negative ink-repellent upper layer type plates are described in JP-B-46-16044, JP-B-61-54222 and JP-A-63-88556, and the negative ink-repellent lower layer type plates are described in JP-B-47-2361, JP-B-56-14976 and JP-A-1-173027. With the recent progress of digitalization of the plate-making process, lithographic printing plates requiring no fountain solution comprising a substrate having thereon a photosensitive layer and an ink-repellent layer have been proposed. Plates comprising silicone rubber as the ink-repellent layer for use in such lithographic printing plates are described in, for example, JP-B-54-26923, JP-B-55-22781, JP-B-56-23150 and JP-A-2-236550. Plates comprising a fluorine resin as the ink-repellent layers are described in, for example, JP-A-58-215411 and JP-A-2-103047. Among them, silicone rubber is more preferably used from the viewpoint of ink repellency.
The silicone rubber layer for use in the lithographic printing plates is usually obtained by partially crosslink-curing a polymer mainly comprising a polysiloxane skeleton with a crosslinking agent. The method for curing the silicone rubber layers includes a condensation method in which a polysiloxane having a hydroxyl group at both ends are crosslinked with a silane compound having at least 3 hydrolytic functional groups directly bonded to a silicon atom to form silicone rubber (e.g., JP-B-56-23150, JP-A-54-54702, JP-A-61-230151).
However, when the condensation silicone rubber layer is prepared by using a highly reactive silane compound as the crosslinking agent, the crosslinking reaction proceeds in the solution state because of the moisture in the air, resulting in gelation of a coating solution for the silicone rubber layer within several hours after solution preparation. Furthermore, when a less reactive silane compound is used as the crosslinking agent, it is disadvantageous because the silicone rubber layer is poor in curing characteristics after coating.